1. Field of the Invention
This invention relates to a gradational printing method for performing a gradational printing by using heat emitting resistors (namely, heating elements) arranged on a thermal printing head (hereunder sometimes referred to simply as a thermal printhead or a thermal head).
2. Description of The Related Art
A widely-known conventional method for performing a gradational printing on a heat sensitive paper or the like is to print a gradational image on a printing paper through a transfer paper (or directly on a thermally sensitive paper) by applying pulses, the width of each of which corresponds to the weight of each bit of digital data representing gray levels, to a plurality of heating elements linearly arranged.
FIGS. 9(a) to 9(d) are timing charts for illustrating an example of such a conventional gradational printing method. Incidentally, this example is obtained by expanding the number of bits of gradation data up to 4, which data is used in a method disclosed in the Japanese Unexamined Patent Publication (Kokai Tokkyo Koho) Official Gazette No. S57-27771. In case of this method, pulses T0, T1, T2 and T3 are made to correspond to bit 0, bit 1, bit 2 and bit 3 (namely, bit positions 0, 1, 2 and 3 (corresponding to 2.sup.0, 2.sup.1, 2.sup.2 and 2.sup.3, respectively)) of 4-bit gradation data inputted as what is called as head data, respectively. (Incidentally, in the instant application, values indicated by the bit 0, bit 1, bit 2 and bit 3 are referred to simply as 2.sup.0 -bit, 2.sup.1 -bit, 2.sup.2 -bit and 2.sup.3 -bit, respectively.) Further, energizing pulses EN1 to EN4 are generated latching the pulses T0, T1, T2 and T3 serially in response to latch pulses LT. Moreover, a gradational printing is performed by continuously applying all of pulse currents to heating elements. Furthermore, each of the pulse currents is applied thereto substantially simultaneously with the generation of the corresponding energizing pulse. Additionally, in FIG. 9(d), I.sub.TH denotes a head current; and I.sub.M a maximum current.
FIGS. 10(a) to 10(d) are timing charts for illustrating another example of the conventional gradational printing method, which is disclosed in the Japanese Unexamined Patent Publication (Kokai Tokkyo Koho) Official Gazette No. S63-1559. (Incidentally, preheating pulses are not shown in these figures.) As is apparent from these figures, in case of this method, the pulses T0, T1, T2 and T3 are applied to heating elements intermittently. Further, each of these pulses is applied thereto substantially simultaneously with the generation of the corresponding energizing pulse.
Meanwhile, in cases of the aforesaid printing methods, if the value of a head current at the time of energizing all of the heating elements of the thermal head simultaneously is I.sub.M, a power supply for the head should be able to supply the current of I.sub.M thereto without variation in voltage for a period of (T0+T1+T2+T3) (namely, a sum of the pulse durations of the pulses T0, T1, T2 and T3) in case of the method of FIGS. 9(a) to 9(d) (incidentally, in case of the method of FIGS. 10(a) to 10(d), for the longest pulse duration (namely, T3) among those of the pulses T0 to T3). As the result, the power supply for the head should have large capacity. Consequently, the size and manufacturing cost of a printer employing such a conventional method should be large. The present invention is accomplished to eliminate the drawbacks of the conventional methods.
It is, therefore, an object of the present invention to provide a gradational printing method which energizes each of blocks of heat emitting elements of a thermal printhead correspondingly to a different bit of gradation data representing a gray level.